1. FIELD OF THE INVENTION
The present invention relates to a method of precise geometric modeling of folded subsurface geological formations, and more particularly, a modeling method based on surveys of formation dip and of the variations of the dip as recorded in holes vertically or directionally bored through said formations by a dipmeter tool.
2. DESCRIPTION OF THE PRIOR ART
Accumulated sediments, which are originally laid in horizontal or sub-horizontal layers, can become folded with time and with changes of lateral and vertical stress to create folds of various sizes and shapes. The folds may create a shape that is generally conical. This conical folding may be visualized as a plurality of nested cups with an essentially horizontal plane passing through the center axis of each of the cups. Where stress exceeds certain points of rupture, faults appear and complicate the folded configuration. Such folds and faults may be shown in surface geological surveys and maps, and in rock outcrops as on the sides of scarps in mountainous regions. Geologists infer the three dimensional geometry of these structures by extrapolating data from surface geological surveys. These extrapolations are conjectural by nature and are valid only over a skin of earth's surface with a thickness on the order of a fraction of a mile.
Subsurface geophysical surveys, such as seismic surveys, permit a deeper penetration into the earth's crust and, consequentially, allow for more interpolation. However, these subsurface surveys also depend on certain assumptions such as the distribution of acoustic velocities in the volume of sediments being investigated, the amount and mode of refraction through these sediments, and the need to "migrate" reflection points where formation dip becomes important. Seismic waves are bent by reflectors which are rocks or sedimentary layers with different densities. Migration reconstitutes the wave path reflections through the sedimentary layers. Subsurface surveys may also be blind to important structural events located below strong such reflectors as subsurface basalt flows.
Well surveys can offer a precise and intimate view or "look" at subsurface sediments. The physical properties of these sediments can be measured on a foot-by-foot basis. These measurements are taken from a hole that is bored through the sediments. One type of well survey is known as a dipmeter survey, which is the survey of slopes, or.the dips, of sediment beds at where they intercept the borehole. A dipmeter survey is made up of a plurality of indicators that show direction (e.g., azimuth) and inclination of a formation surface intersecting the line of the wellbore.
A survey system using the output of a dipmeter tool is disclosed in U.S. Pat. No. 4,414,656. A dipmeter tool is suspended within a wellbore and is moved through the wellbore course to produce electrical signals representative of the subsurface formations through which the wellbore penetrates. The dipmeter tool records electrical or other types of signals from directionally sensitive sensors spaced radially along the tool.
Dipmeter surveys offer a precise measurement of dip on a near continuous basis along a borehole. In general, dip varies in a continuous manner over hundreds or thousands of meters. Graphical displays of measurements taken at one-foot increments form patterns which can then be loosely classified according to their geometry. These patterns are interpreted in terms of subsurface structural configurations with a view to extrapolate the configurations at some distance from the borehole.
Extrapolating such patterns has previously been done in a qualitative, "hand-waving" manner. This hand-waving manner describes a technique for the approximate interpretation of the subterranean surfaces. Additional constraints are required to extrapolate on a sound qualitative basis. An accepted constraint is the conservation of bed thickness, which accounts for the conservation of bed volume. Another difficulty in the interpretation of dip patterns is the irregular course of the borehole through the formations. Though most boreholes in the past had a substantially vertical orientation, in recent years, directional drilling has become more commonplace. Directional drilling can achieve boreholes with a high angle of deviation from the vertical axis, and even horizontal drilling is not uncommon. Prior dipmeter techniques do not account for such borehole deviations in the interpretation of borehole patterns.
The following patents describe methods of processing and interpreting dipmeter surveys. Of the following patents, U.S. Pat. Nos. 4,873,636; 4,852,005; 4,357,660; 4,348,748; 4,303,975 were all issued to the instant inventor. U.S. Pat. Nos. 4,873,636; 4,852,005; and 4,414,656 are hereby incorporated by reference into the instant application.
U.S. Pat. No. 4,942,528 issued to Mark G. Kerzner on Jul. 17, 1990, describes a method for processing a dipmeter curve using a segmentation tree to represent the curve. The segmentation tree is converted into an event tree by deleting curve events falling outside certain event criteria. Correlation coefficients are determined and optimized between pairs of curves using the event tree, and formation dip is determined from optimized correlation curves.
U.S. Pat. No. 4,939,649 issued to John A. Duffy et al. on Jul. 3, 1990, describes a method of correcting nonunimodiality of dipmeter traces. Dipmeter data comprises nonunimodial datasets which are transformed into nonunimodial-symmetric datasets, while the subsets that are already nonunimodial-symmetric are maintained.
U.S. Pat. No. 4,873,636 issued to the instant inventor, Vincent R. Hepp on Oct. 10, 1989, describes a method of interpreting conical structures from dipmeter surveys. The dip modeling disclosed is restricted to vertical holes drilled in conical structures.
U.S. Pat. No. 4,853,855 issued to Mark G. Kerzner on Aug. 1, 1989, describes a method for processing a dipmeter curve where line segments are drawn between curve minima to create a segmentation tree. The segmentation tree is reorganized to form an event tree which is easily converted into a stored digital value and processed for correlation with other curves.
U.S. Pat. No. 4,852,005 issued to Vincent R. Hepp et al. on Jul. 25, 1989, describes a method of computing formation dip and azimuth wherein portions of at least three dipmeter surveys are matched to derive a plurality of possible offsets for defining a plurality of dips.
U.S. Pat. No. 4,414,656 issued to Vincent R. Hepp on Nov. 8, 1983, describes a well logging system for mapping structural and sedimentary dips of underground earth formations. The dips are identified by the depth at which it occurs, its dip magnitude angle, its dip azimuth angle, and the cell in a hemispherical equal area map to which the dip belongs.
U.S. Pat. No. 4,357,660 issued to Vincent R. Hepp on Nov. 2, 1982, describes a formation dip and azimuth processing technique in which dip and azimuth variations over a given interval are used to define a family of surfaces in a three dimensional reference system.
U.S. Pat. No. 4,348,748 issued to Christian M. J. Clavier et al. on Sep. 7, 1982, describes a dipmeter displacement processing technique that allows a processor to derive the most probable value of formation dip from a set of curve displacements derived from a dipmeter survey.
U.S. Pat. No. 4,303,975 issued to Vincent R. Hepp on Dec. 1, 1981, describes a dipmeter displacement qualifying technique.
While these and other references disclose methods of modeling based on dipmeter surveys, the known prior art does not disclose or suggest a method using dip modeling for all structures of constant or near constant bed thickness, and to any borehole course. For example, none described a method for accounting for borehole deviation in the interpretation of dip patterns. There currently exists a need for a precise description of subsurface geological structures based on a continuous survey of formation dip through vertical or deviated borehole. None of the above references, either alone or in combination with one another, is seen to describe the instant invention as claimed.